The invention relates generally to wireless communication, and more particularly to wireless transmission of data from mobile data acquisition units to a stationary central receiver.
Wireless services are increasingly ubiquitous and useful components in the global communication infrastructure, and wireless data telemetry is widely used because it allows collection of sensor information from any location in an antenna coverage area without reconfiguration of the communications infrastructure. An example of particular importance in medical practice is the wireless transmission of the electrocardiogram (ECG) data and other physiological monitoring signals acquired from patients in a hospital environment. Wireless telemetry allows these patients to be mobile, while vital signs of the patients are continuously monitored.
Many wireless telemetry systems are structured to employ frequency division multiple access (FDMA) schemes. In an FDMA transmission, each transmitter uses only a small band of contiguous frequencies, and frequency bands assigned to different transmitters are disjoint. All of the FDMA channels are typically contained within a larger band of frequencies, which is usually called “the channel”.
Furthermore, wireless communication systems often operate in environments with severe fading due to multi-path propagation, which limits system performance. In the context of FDMA telemetry systems, such fading channels have the effect of transmitting each of the FDMA channels with a different power, and they are generally called multi-path fading channels. In general, this frequency response characteristic of the channel will change over time, due primarily to motion of the mobile transmitter, but also due to motion of other objects in the environment. In particular, one such frequency-selective fading environment is the indoor radio transmission environment. The wireless transmission of ECG telemetry data falls into this category, since it typically occurs inside a hospital building. A simple approach to overcoming the effects of fading is to provide for additional radiated power at the transmitter, over and above the power required to achieve the specified bit error rate (BER) at the specified range. Alternative schemes that do not depend on high transmit power may be advantageous in that they can, under some circumstances, translate into increased channel capacity. One such alternative scheme used to mitigate the effects of fading is the use of spatial antenna diversity.
Digital antenna arrays are of great interest to wireless communication systems. By utilizing spatial antenna diversity, the potential for performance improvement in wireless systems is vast. As will be appreciated, the spatial diversity approach employs multiple receive antennas to generate multiple copies of the same information-bearing signal. These copies are then combined in some fashion prior to demodulation of the received signal. This can help the system combat both multi-path fading and blockage of the radio frequency (RF) signal by obscuring objects (such as elevator shafts).
However, in a system that uses a spatial diversity scheme, it is often necessary to select the “best” receiving antenna field to use in the receiver. The selection criteria may be based on the highest signal power received or highest estimated signal-to-noise ratio (SNR). When using such “selection combining”, the system performance may experience degradation due to loss of data experienced during antenna switching. Additionally, if the rate at which the antennas are switched is not high enough, changes in the environment may not be adequately tracked, resulting in temporary increases in the BER of the demodulated information.
A desirable alternative to antenna selection combining is coherent combination of the signals by the well-known technique of maximal ratio combining. In this combination scheme, signals are weighted by their measured received signal strength and the estimated noise power in the receive channel. This scheme results in a much better output signal-to-noise ratio than does selection combining. The most convenient form for such a combination is one in which all the signals are demodulated to their complex baseband representation prior to weighting and summation, although the combination may also be performed if the signals are modulated to a common intermediate frequency.
In order to coherently combine two received versions of a single FDMA channel, both of the antenna output signals at the two separate antennas must be channelized, translated to baseband, the required signal and noise powers must be estimated and the two signals “time-aligned”, weighted and summed. In the past, the most practical approach to this sequence of operations was to perform the channelization in analog hardware, translate to baseband using analog mixers and local oscillators, digitize the result and perform the power estimation and summation in software or firmware. This approach requires multiple analog front-ends, one for each antenna. This is to be contrasted with selection combining, which requires only an analog switch to connect the selected antenna to the front-end hardware. It is because of this difference in implementation cost that selection combining is more commonly used than coherent combining.
More recently, the availability of very high speed analog-to-digital converters (ADCs) and digital signal processors (DSPs) has sparked interest in the use of software to perform many radio receiver functions that were formerly done in analog hardware.
It may therefore be desirable to develop an approach to coherent combination of signals in a FDMA telemetry radio that advantageously facilitates enhanced performance of the wireless communication systems in a multi-path fading environment.